This invention relates generally to spin valves. More particularly, it relates to a structure of high resistivity ferromagnetic films for AP layers in spin valves.
A spin valve or a magnetoresistive (MR) sensor detects magnetic field signals through the resistance changes of a read element, fabricated of a magnetic material, as a function of the strength and direction of magnetic flux being sensed by the read element. The conventional MR sensor operates on the basis of the anisotropic magnetoresistive (AMR) effect in which a component of the read element resistance varies as the square of the cosine of the angle between the magnetization in the read element and the direction of sense current flow through the read element. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the read element, which in turn causes a change in resistance (xcex94R/R) in the read element and a corresponding change in the sensed current or voltage.
A spin valve has been identified in which the resistance between two uncoupled ferromagnetic layers is observed to vary as the cosine of the angle between the magnetizations of the two layers and is independent of the direction of current flow. The spin valve produces a magnetoresistance that, for selected combinations of materials, is greater in magnitude than the AMR. In general, the larger xcex94R/R is the better the spin valve""s performance.
An external magnetic field causes a variation in the relative orientation of the magnetization of a neighboring ferromagnetic layer. This in turn causes a change in the spin-dependent scattering of conduction electrons and thus the electrical resistance of the spin valve. The resistance of the spin valve thus changes as the relative alignment of the magnetizations of the ferromagnetic layers changes.
Typically, a conventional spin valve comprises a ferromagnetic free layer, a spacer layer, and a single-layer pinned ferromagnetic layer, which is exchange-coupled with an antiferromagnetic (AF) layer. In an anti-parallel (AP) pinned spin valve, the single-layer pinned ferromagnetic layer is replaced by a laminated structure comprising at least two ferromagnetic pinned sublayers separated by one or more thin anti-ferromagnetic (AF) coupling or AP spacer sublayers. The two ferromagnetic pinned sublayers are antiferromagnetically coupled to one another, by means of the appropriate type and thickness of AF coupling sublayer, so that their magnetizations are oriented anti-parallel to one another. Since the moments are anti-parallel, the net moment of the AP structure is the difference of the net moments of the magnetic sublayers. Thus, the net moment can then be made arbitrarily small. This greatly enhances the pinning of the AP layer, since the pinning between the AP layer and the antiferromagnetic layer is inversely proportional to the net moment.
A usual spin valve usually has a single or double layered ferromagnetic free layer. The single layer version is typically permalloy. Often a thin layer of cobalt or cobalt alloy is applied to enhance the xcex94R/R of the spin valve. This double layered structure is referred to as a nanolayered structure.
Future high density recording applications require very small net magnetic moments for the free layer. This can be obtained by reducing the physical thickness of the free layer, but xcex94R/R drops. The AP free layer design can be used to offset this loss in xcex94R/R. Just as in the AP pinned layer, the net magnetic moment is reduced without reducing the physical thickness. However, the additional ferromagnetic pinned sublayer and the AP spacer sublayer are conductive, so they tend to shunt the sense current away from the xe2x80x9cactivexe2x80x9d region of the spin valve. This reduces the resistance of the sensor, reducing the voltage obtained for a given current. Further, the shunting effect of the extra layers in an AP structure reduces the number of electrons that undergo spin-dependent scattering by removing them from the active region around the free layer and the first pinned sublayer. Since the signal (xcex94R/R) in a spin valve results from the spin-dependent scattering of electrons, the shunting effect reduces the xcex94R/R value.
U.S. Pat. No. 5,591,533, filed Jun. 1, 1995 discloses a simple spin valve in which a substrate surface layer comprises a metal such as Cr or Ta. The substrate layer has a surface typically comprising Al2O3 or SiO2. Chromium and tantalum are deposited over and contiguous with a top layer of Al2O3 or SiO2. These metals after deposition have high resistivity due to partial oxidation when formed on the substrate. This highly resistive surface layer prevents the electrons from scattering into the substrate layer, which reduces shunting of the sense current in the magnetoresistive element and further acts as a barrier to protect the alloy from interface contamination. Thus, this is a method to increase signal by reducing shunting. However, this technique cannot be used in AP layers, since the oxide layers noted are non-magnetic.
U.S. Pat. No. 5,898,549 filed Oct. 27, 1997 disclosed an anti-parallel (AP) pinned spin valve in which the AP pinned layer includes first, second, and third pinned layers, where the first pinned layer is separated from the second and third pinned layers by an anti-parallel coupling layer. The first and second pinned layers are made of highly resistive material such as NiFeCr, NiFeRh, or NiFeMo, and the third pinned layer is made of low resistive material such as Cobalt. The use of highly resistive first and second pinned layers reduces the amount of sense current flowing in the AP-pinned layer. However, this technique teaches the use of only NiFeCr, NiFeRh, and NiFeMo to make the highly resistive pinned layer, and does not teach a highly resistive AP ferromagnetic free layer. In addition, the NiFe-based alloys have lower magnetizations than the Cobalt alloys. In order to obtain a target value for the moment of a sublayer, the NiFe-based alloys need to be thicker, negating some of the advantages of the high resistivity of the layers. Cobalt has a moment 1.7 times the value of NiFe. Thus, a Co-based alloy can be 1.7 times thinner than its NiFe-based counterpart. Given similar resistivities, the cobalt alloys will shunt 1.7 times less current, a marked improvement over the NiFe alloys. Furthermore, U.S. Pat. No. 5,898,549 does not discuss the effect of the highly resistive layer on xcex94R/R value.
There is a need, therefore, for high resistivity ferromagnetic films for use in the AP pinned and/or AP free layers of spin valves. These will reduce the net magnetic moment of the free and/or pinned layers with less reduction in xcex94R/R that would occur if the physical thickness of the layers were simply reduced.
Accordingly, it is a primary object of the present invention to provide a structure for spin valves with highly resistive films for AP pinned layer and AP free layer that:
1) can increase the overall sheet resistance of the spin valve
2) can reduce the shunting of the sense current
3) can optimize the xcex94R/R value of the spin valve
These and other objects and advantages will be apparent upon reading the following description and accompanying drawings.
These objects and advantages are attained by a spin valve that detects an external magnetic field with the aid of high resistivity films for AP layers in the spin valve. The high resistive layer can be within an AP pinned layer and/or an AP free layer of the spin valve.
Typically, a simple spin valve comprises an anti-ferromagnetic (AF) layer, a pinned ferromagnetic layer, a ferromagnetic free layer, and a non-magnetic spacer layer. In an AP spin valve the pinned layer, the free layer or both are replaced by a trilayer, AP structure comprising a first ferromagnetic sublayer, a nonmagnetic AP spacer sublayer and a second ferromagnetic sublayer. An AP spin valve typically has lower xcex94R/R value than a simple spin valve, because the extra ferromagnetic sublayer and AP spacer sublayer are conductive, and therefore they shunt the sense current away from the rest of the structure of spin valve. The shunting effect of the extra layers in an AP structure reduces the number of electrons that undergo spin-dependent scattering by removing them from the xe2x80x9cactivexe2x80x9d region around the free layer and the first pinned sublayer. The signal (xcex94R/R) in a spin valve, which results from spin dependent scattering of electrons, is reduced due to the shunting loss from the additional layers.
In accordance with a first embodiment of the present invention, the AP pinned ferromagnetic layer comprises a first ferromagnetic pinned sublayer, a second ferromagnetic pinned sublayer, and an AP spacer sublayer between the first and second ferromagnetic pinned sublayer. One of the two ferromagnetic pinned sublayers of the AP pinned layer is a highly resistive layer that may include a highly resistive alloy of the type AB, wherein A is selected from the group consisting of NiFe, CoFe, or CoFeNi, and B is selected from the group consisting of B, Ta, Nb, Zr, and/or Hf.
In accordance with a second embodiment of the present invention, the AP ferromagnetic free layer of a simple spin valve comprises a first ferromagnetic free sublayer, an AP coupling nonmagnetic sublayer, and a second ferromagnetic free sublayer. One of the two ferromagnetic free sublayers of the AP free layer is a highly resistive layer that includes similar high resistivity materials to the AP pinned sublayer in the first embodiment of the present invention.
In accordance with a third embodiment of the present invention, either or both of the AP ferromagnetic free layer and the AP pinned layer of a spin valve include a highly resistive sublayer that includes the similar high resistivity materials to the AP pinned sublayer in the first embodiment of the present invention.
In accordance with a fourth embodiment of the present invention, a dual spin valve comprises two AP ferromagnetic pinned layers located symmetrically about a ferromagnetic free layer. Either or both of the AP ferromagnetic pinned layers include a highly resistive sublayer that includes the similar high resistivity materials to the AP pinned sublayer in the first embodiment of the present invention.
The high resistivity materials of the AP pinned layer and/or AP free layer reduce the shunting effect, thereby optimizing the xcex94R/R value.